This invention relates to drilling apparatus and in particular to drilling apparatus for determining information about rock formations about a borehole.
When drilling, and particularly when drilling for oil, the design of the well-bore trajectory will largely be based on the interpretation of data acquired by a surface seismic survey. One drawback with data obtained by such a survey is that the depth of sub-surface features such as rock boundaries are not defined in terms of distances but in terms of the time it takes an acoustic signal to travel to the feature from the surface and back again. In order to aid the interpretation of the seismic data it is therefore desirable to relate these acoustic travel times to actual depths thus establishing a xe2x80x98time-depth tiexe2x80x99.
Conventionally this is performed by removing the drill string from the well-bore and deploying an acoustic receiver into the well on a wireline. A source on the surface is used to transmit acoustic energy into the earth, and the acoustic signal is detected by the down-hole receiver. By knowing the depth of the receiver and by measuring the time it takes the acoustic signal to travel from the surface to the receiver, the time-depth relationship can be established for this particular depth. This process is repeated with the receiver located at several different depths. Such a method is referred to as a xe2x80x98check-shotxe2x80x99 survey. Removal of the drill string from a well-bore is time consuming and expensive.
It is an object of the present invention to provide apparatus by which the time-depth tie xe2x80x98check-shotxe2x80x99 measurement may be made without the need to remove the drilling apparatus from the borehole.
Also of interest whilst drilling is information about the rock strata that will be drilled through at any time, in order to allow appropriate drilling parameters and strategies to be employed. One particular concern is the presence of overpressure regions in the strata ahead of the drill which have the potential for blowing into the well when penetrated, potentially creating a hazardous situation. This situation may be alleviated by appropriately altering the weight or pressure of the drilling mud which is present in the well-bore. Another concern is the setting of casing in the correct place in the well-bore, which may have implications regarding the ability of the well to produce oil. A still further concern is in the drilling along a reservoir formation, which may be at or near to horizontal, without breaking through into the formations above or below the reservoir. All of these concerns require knowledge of the rock formations about the drill bit and thus the borehole.
Patent Application GB 2288020 discloses drilling apparatus in which an acoustic source and receiver arrangement is mechanically connected to a drill bit at the end of a drill string in a borehole. Acoustic signals are transmitted by way of the drill bit into the rock formations ahead of the bit, with reflected signals being received again through the drill bit. In order to achieve the required resolution, the acoustic signals used in this apparatus will be above the frequency range of conventional surface seismic exploration apparatus. However, the use of these higher frequencies limits the range of this apparatus and makes a direct comparison with surface seismic data, to establish a tie between the position of the drill bit and the surveyed formations, difficult.
The object of the invention is to provide improved drilling method and apparatus.
In accordance with a first aspect of the present invention, there is provided drilling apparatus for determining information about the rock formations about a borehole, comprising:
a drill bit connected at the end of a drill string in the borehole and in contact with rock at the end of the borehole;
an acoustic source located at or near the earth""s surface; and
an acoustic receiver in a rigid mechanical connection with the drill bit such that acoustic signals transmitted by the acoustic source and propagated through the formations are received by the acoustic receiver via the drill bit; wherein
the acoustic receiver is a piezo-ceramic transducer having one face in rigid connection with the drill bit; and
a face of said acoustic receiver opposite the drill bit is connected to a tail mass.
Travel time-to-depth relationships may be determined by measurement of the propagation time of acoustic signals from the source to the receiver. In this case, the information about the rock formations is the characteristics of acoustic propagation through them. Alternatively or in addition, the existence and positions of boundaries between different rock types, other materials or other such parameter may be determined. The present invention allows information about the rock formations to be obtained without requiring the removal of the drill string and bit.
In accordance with a second aspect of the present invention, there is provided a method of obtaining information about the rock formations about a borehole, comprising:
(a) drilling the borehole with a drill bit connected to the end of a drill string;
(b) transmitting an acoustic signal from at or near the earth""s surface;
(c) receiving the acoustic signal with an acoustic receiver in a rigid mechanical connection with the drill bit, the acoustic signal travelling through the rock formations and the drill bit to the acoustic receiver; and
(d) recording measurement data obtained by the acoustic receiver.
Preferably this method further comprises: repeating steps (a) to (d) such that measurement data is recorded in respect of a different position of the drill bit.
This aspect of the invention may allow check shot information and/or information equivalent to a Vertical Seismic Profile to be obtained without requiring the drill string and bit to be removed from the borehole.